Image-pickup optical systems, which are used not only for the visible-light range but also for the infrared-frequency range or the terahertz-frequency range, have been developed as image-pickup optical systems configured to detect images.
For example, an infrared image-pickup optical system uses heat from an object such as a person or an animal (i.e., far-infrared (wavelength 8 μm to 12 μm)), and is used to pick up an image in a dark place, to observe a temperature distribution, or the like.
Moreover, an image-pickup optical system for the terahertz wave (wavelength 30 μm to 3 mm: 100 GHz to 10 THz) is used for so-called non-destructive tests such as security checks at airport facilities.
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